Many types of implantable medical devices (IMDs) have been clinically implanted over the last twenty years that deliver relatively high-energy cardioversion and/or defibrillation shocks to a patient's heart when a malignant tachyarrhythmia, e.g., atrial or ventricular fibrillation, is detected. Cardioversion shocks are typically delivered in synchrony with a detected R-wave when fibrillation detection criteria are met, whereas defibrillation shocks are typically delivered when fibrillation criteria are met and an R-wave cannot be discerned from the EGM.
The current state of the art of ICDs or implantable pacemaker/cardioverter/defibrillators (PCDs) includes a full featured set of extensive programmable parameters which includes multiple arrhythmia detection criteria, multiple therapy prescriptions (for example, stimulation for pacing in the atrial, ventricular and dual chamber; atrial and ventricular for bradycardia; bi-atrial and/or bi-ventricular for heart failure; and arrhythmia overdrive or entrainment stimulation) and high level stimulation for cardioversion and/or defibrillation, extensive diagnostic capabilities and high speed telemetry systems. These full-featured ICDs or PCDs, hereinafter IMD, are typically implanted into patients who have had, and survived, a significant cardiac event (such as sudden death). Additionally, these devices are expected to last up to 5-8 years and/or provide at least 200 life saving therapy shocks.
Even though there have been great strides in size reduction over the past 20 years, the incorporation of all these features in an IMD, including the longevity requirements, dictates that the devices be typically much larger than current state of the art pacemakers. Such devices are often difficult to implant in some patients (particularly children and thin, elderly patients) and typically require the sacrifice of 1 or 2 veins to implant the lead system because leads associated with the implantation of an IMD utilize a transvenous approach for cardiac electrodes and lead wires. The defibrillator canister/housing is generally implanted as an active can for defibrillation and electrodes positioned in the heart are used for pacing, sensing and detection of arrhythmias.
Although IMDs and implant procedures are very expensive, most patients who are implanted have experienced and survived a sudden cardiac death episode because of interventional therapies delivered by the IMDs. Survivors of sudden cardiac death episodes are in the minority, and studies are ongoing to identify patients who are asymptomatic by conventional measures but are nevertheless at risk of a future sudden death episode. Current studies of patient populations, e.g., the MADIT II and SubQ ICD HeFT studies, are establishing that there are large numbers of patients in any given population that are susceptible to sudden cardiac death, that they can be identified with some degree of certainty and that they are candidates for a prophylactic implantation of a defibrillator (often called primary prevention). However, implanting currently available IMDs in all such patients would be prohibitively expensive. Further, even if the cost factor is eliminated there is shortage of trained personnel and implanting resources.
One option proposed for this patient population is to implant a prophylactic subcutaneous implantable cardioverter/defibrillator (SubQ ICD) such that when these patients receive a shock and survive a cardiac episode, they will ultimately have an implant with a full-featured ICD and transvenous leads. The housing or “can” of the device includes a first shield and a second shield that are joined together by a weld formed along a seam following placement of the internal components within shields to seal device, so that the two shields define an enclosure for the internal components of device. Since the housing is typically positioned subcutaneously in close proximity to the patient's ribs, for example, current configurations for the housing for such devices include the use of non-planar surfaces, such as a concave surface, for example, in order to better enable positioning of the housing in the patient. As a result, there is a need for an improved method and apparatus for forming the weld along the seam of the housing that addresses the non-planar seam weld profile.